Frictional forces, such as
fsize 12{f} {} , always oppose motion or attempted motion between objects in contact. Friction arises in part because of the roughness of the surfaces in contact, as seen in the expanded view. In order for the object to move, it must rise to where the peaks can skip along the bottom surface. Thus a force is required just to set the object in motion. Some of the peaks will be broken off, also requiring a force to maintain motion. Much of the friction is actually due to attractive forces between molecules making up the two objects, so that even perfectly smooth surfaces are not friction-free. Such adhesive forces also depend on the substances the surfaces are made of, explaining, for example, why rubber-soled shoes slip less than those with leather soles.

The magnitude of the frictional force has two forms: one for static situations (static friction), the other for when there is motion (kinetic friction).

When there is no motion between the objects, the
magnitude of static friction
fssize 12{f rSub { size 8{s} } } {} is

fs≤μsN,size 12{f rSub { size 8{s} }<= μ rSub { size 8{s} } N} {}

where
μssize 12{μ rSub { size 8{s} } } {} is the coefficient of static friction and
N is the magnitude of the normal force
(the force perpendicular to the surface).

Magnitude of static friction

Magnitude of static friction
fssize 12{f rSub { size 8{s} } } {} is

fs≤μsN,size 12{f rSub { size 8{s} }<= μ rSub { size 8{s} } N} {}

where
μssize 12{μ rSub { size 8{s} } } {} is the coefficient of static friction and
N is the magnitude of the normal force.

The symbol
≤size 12{<= {}} {} means
less than or equal to , implying that static friction can have a minimum and a maximum value of
μsNsize 12{μ rSub { size 8{s} } N} {} . Static friction is a responsive force that increases to be equal and opposite to whatever force is exerted, up to its maximum limit. Once the applied force exceeds
fs(max)size 12{f rSub { size 8{s \( "max" \) } } } {} , the object will move. Thus

Once an object is moving, the
magnitude of kinetic friction
fksize 12{f rSub { size 8{k} } } {} is given by

fk=μkN,size 12{f rSub { size 8{k} } =μ rSub { size 8{k} } N} {}

where
μksize 12{μ rSub { size 8{K} } } {} is the coefficient of kinetic friction. A system in which
fk=μkNsize 12{f rSub { size 8{k} } =μ rSub { size 8{k} } N} {} is described as a system in which
friction behaves simply .

As seen in
[link] , the coefficients of kinetic friction are less than their static counterparts. That values of
μsize 12{μ} {} in
[link] are stated to only one or, at most, two digits is an indication of the approximate description of friction given by the above two equations.

Coefficients of static and kinetic friction

System

Static friction
μssize 12{μ rSub { size 8{s} } } {}

Kinetic friction
μksize 12{μ rSub { size 8{K} } } {}

Rubber on dry concrete

1.0

0.7

Rubber on wet concrete

0.7

0.5

Wood on wood

0.5

0.3

Waxed wood on wet snow

0.14

0.1

Metal on wood

0.5

0.3

Steel on steel (dry)

0.6

0.3

Steel on steel (oiled)

0.05

0.03

Teflon on steel

0.04

0.04

Bone lubricated by synovial fluid

0.016

0.015

Shoes on wood

0.9

0.7

Shoes on ice

0.1

0.05

Ice on ice

0.1

0.03

Steel on ice

0.4

0.02

The equations given earlier
include the dependence of friction on materials and the normal force. The direction of friction is always opposite that of motion, parallel to the surface between objects, and perpendicular to the normal force. For example, if the crate you try to push (with a force parallel to the floor) has a mass of 100 kg, then the normal force would be equal to its weight,
W=mg=(100 kg)(9.80m/s2)=980 Nsize 12{W="mg"= \( "100""kg" \) \( 9 "." "80"`"m/s" rSup { size 8{2} } \) ="980"N} {} , perpendicular to the floor. If the coefficient of static friction is 0.45, you would have to exert a force parallel to the floor greater than
fs(max)=μsN=0.45(980N)=440Nsize 12{f rSub { size 8{S \( "max" \) } } =μ rSub { size 8{S} } N=0 "." "45" times "980"N="440"N} {} to move the crate. Once there is motion, friction is less and the coefficient of kinetic friction might be 0.30, so that a force of only 290 N(
fk=μkN=0.30980N=290Nsize 12{f rSub { size 8{k} } =μ rSub { size 8{k} } N= left (0 "." "30" right ) left ("980"" N" right )="290"" N"} {} )
would keep it moving at a constant speed. If the floor is lubricated, both coefficients are considerably less than they would be without lubrication. Coefficient of friction is a unit less quantity with a magnitude usually between 0 and 1.0. The coefficient of the friction depends on the two surfaces that are in contact.

Questions & Answers

can a given total amount of mechanical energy be totally converted into heat energy..if so give example

evaporation is the process of extracting moisture while vaporization is process of becoming a vapor or gas

Emmanuel

From a molecular standpoint they are both cooling processes.
Also, you may want to explore states of matter😊
#myTwoCents
~Shi~

Shii

cooling is a similarlity in both process I am confused in difference

Muhammad

1- Evaporation is a process where a liquid change to gas without reaching its boiling point.
2- Vaporization is a process where a liquid change to gas after reaching its boiling point.
3- Sublimation is a process where a solid changes into vapour without passing through a liquid state

Victor

I see. Evaporation is a type of vaporization, that occurs on the surface of a liquid as it changes into the gaseous phase before reaching its boiling point.
hope that aids

Shii

vaporisation is cooling process while vaporization is heating process

Emmanuel

I mean to write evaporation is an heating process while vaporization is cooling process

Emmanuel

Yea here are two applications.
1- your wet washed clothes dry under the sun, the water EVAPORATES
2- when u are cooking, it reaches a point where u need to add more water because the water you added previously is getting dried. this is VAPORIZATION. Am not sure which is a cooling or heating process

Victor

vaporization occur only when the evaporation get to level where the above cloud is been (saturated) so cooling take place and started to change to liquid (eg rain fall)

Emmanuel

They are both properties of the same process so they're both cooling

Shii

what about sublimation? cooling or heating process?

Victor

exact

Muhammad

evaporation is the increase in kinetic energy of the liquid which can be gone by adding heat

Emmanuel

so its an heating process

Emmanuel

sublimation is when a solid change to gas

Emmanuel

evaporation is very definitely a cooling process.
respectfully@Emmanuel
when liquid turns to gas it requires more energy from its surroundings, this energy is in the form of heat, and when heat energy leaves the evaporating liquid it leaves it cooler. Thus, cooling process.

Shii

.

Shii

evaporation is very definitely a cooling process.
respectfully@Emmanuel

Shii

kk

Emmanuel

You're right @Shi. I get your point

Victor

eascape velocity on the surface of Earth is 11.2 kms-1 the escape velocity on the surface of another planet of same mass as that of Earth but of 1/4 times of radius of Earth is
a5.6kms-1 b11.2 kms-1 c22.4kms-1 d5.6ms-1

A petrol engine has a output of 20 kilowatts and uses 4.5 kg of fuel for each hour of running. The energy given out when 1 kg of petrol is burnt is 4.8 × 10 to the power of 7 Joules.
a) What is the energy output of the engine every hour?
b) What is the energy input of the engine every hour?

A simple pendulum is used in a physics laboratory experiment to obtain an experimental value for the gravitational acceleration, g . A student measures the length of the pendulum to be 0.510 meters, displaces it 10 o from the equilibrium position, and releases it. Using a s

Two masses of 2 kg and 4 kg are held with a compressed spring between them. If the masses are released, the spring will push them away from each other. If the smaller mass moves off with a velocity of 6m/s, what is the stored energy in the spring when it is compressed?

Reduce that two body problem into one body problem. Apply potential and k. E formula to get total energy of the system

rakesh

i dont think dere is any potential energy... by d virtue of no height present

Olalekan

there is compressed energy,dats only potential energy na?

rakesh

yes.. but... how will u approach that question without The Height in the question?

Olalekan

Can you explain how you get 54J?

Emmanuel

Because mine is 36J

Emmanuel

got 36J too

Douglas

OK the answer is 54J
Babar is correct

Emmanuel

Conservation of Momentum

Emmanuel

woow i see.. can you give the formula for this

joshua

Join the discussion...

Two masses of 2 kg and 4 kg are held with a compressed spring between them. If the masses are released, the spring will push them away from each other. If the smaller mass moves off with a velocity of 6m/s, what is the stored energy in the spring when it is compressed? Asume there is no external force.

we are given f=12 m=200g which is 0.2kg
now from 2nd law of newton a= f/m=60m/s*2
work done=force applied x displacement cos (theta)
w= 12x60 =720nm/s*2

Mudang

this very interesting question very complicated for me, í need urgent help.
1,two buses A and B travel along the same road in the same direction from Harper city (asume They both started from the same point) to Monrovia. if bus A maintains a Speedy of 60km/h and bus B a Speedy of 75km/h, how many

mohammed

hours Will it take bus B to overtake bus A assuming bus B starts One hour after bus A started. what is the distance travelled by the buses when They meet?.

mohammed

pls í need help

mohammed

4000 work is done

Ana

speed=distance /time
distance=speed/time

Ana

now use this formula

Ana

what's the answer then

Julius

great Mudang

Kossi

please Ana
explain 4000 ?

babar

hey mudang
there is a product of force and acceleration not force and displacement

babar

@Mohammed answer is 0.8hours or 48mins

Douglas

nice

A.d

its not possible

Olalekan

í want the working procedure

mohammed

the answer is given but how Will One arrive at it. the answers are 4hours and 300m.